There is no vaccine to prevent human visceral leishmaniasis (VL) and the efficacy of conventional antileishmanial chemotherapy is declining. Therefore, to address this state and develop new treatment strategies, this application focuses exclusively on the immunoregulatory mechanisms of the successful host response to antileishmanial chemotherapy. This is a critical research area with clear-cut future as well as already realized clinical impact in this systemic protozoal human infection. This project's overall objective is to understand and then learn how to experimentally manipulate these mechanisms in vivo in favor of the host receiving chemotherapy. The unifying hypothesis tested is that key immunoregulatory mechanisms can be specifically targeted and simultaneously joined in parasitized tissues with the effect of chemotherapy to enhance the killing of intracellular Leishmania donovani and thereby optimize the host response to treatment. To accomplish this objective, in vivo responses to two clinically appropriate chemotherapeutic probes, antimony (Sb) and amphotericin B, will be used to test basic hypotheses to identify critical immunoregulation of the successful response to treatment. Host defense and chemotherapy will be tested at four most import intersections, at the levels of : (a) the basic antileishmanial pathways of the mononuclear phagocyte, the target host defense cell, (b) effector monocyte influx and action in parasitized tissue, (c) Key activating cytokines, and (d) in the intact animal with uncontrolled visceral infection. Our specific Aims are to: 1. Determine if toxic nitrogen and/or oxygen intermediates, specific macrophage leishmanicidal mechanisms, are required molecules for Sb-induced intracellular killing. 2. Test if influx of Sb-accumulating monocytes into parasitized tissue granulomas is a determinant of successful chemotherapy and use granuloma-deficient, Sb-unresponsive mutant mice to conform this hypothesis. 3. Characterize interleukin 12 (IL- 12) in the immunoregulation of the response to chemotherapy and determine if a novel IL-12-induced interferon-gamma-independent mechanism augments Sb efficacy. 4. Characterize endogenous host defense mechanisms in the leishmanicidal action of amphotericin B and develop immunologic enhancement of amphotericin B's efficacy. And 5. Test the rational application of combination immunochemotherapy in a model of uncontrolled visceral infection induced by a clinically relevant Th2 cell-associated state.